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This module defines base classes for standard Python codecs (encoders and
decoders) and provides access to the internal Python codec registry, which
manages the codec and error handling lookup process. Most standard codecs
are text encodings, which encode text to bytes,
but there are also codecs provided that encode text to text, and bytes to
bytes. Custom codecs may encode and decode between arbitrary types, but some
module features are restricted to use specifically with
text encodings, or with codecs that encode to
bytes.

The module defines the following functions for encoding and decoding with
any codec:

Errors may be given to set the desired error handling scheme. The
default error handler is 'strict' meaning that encoding errors raise
ValueError (or a more codec specific subclass, such as
UnicodeEncodeError). Refer to Codec Base Classes for more
information on codec error handling.

Errors may be given to set the desired error handling scheme. The
default error handler is 'strict' meaning that decoding errors raise
ValueError (or a more codec specific subclass, such as
UnicodeDecodeError). Refer to Codec Base Classes for more
information on codec error handling.

Looks up the codec info in the Python codec registry and returns a
CodecInfo object as defined below.

Encodings are first looked up in the registry’s cache. If not found, the list of
registered search functions is scanned. If no CodecInfo object is
found, a LookupError is raised. Otherwise, the CodecInfo object
is stored in the cache and returned to the caller.

The stateless encoding and decoding functions. These must be
functions or methods which have the same interface as
the encode() and decode() methods of Codec
instances (see Codec Interface).
The functions or methods are expected to work in a stateless mode.

Incremental encoder and decoder classes or factory functions.
These have to provide the interface defined by the base classes
IncrementalEncoder and IncrementalDecoder,
respectively. Incremental codecs can maintain state.

Register a codec search function. Search functions are expected to take one
argument, being the encoding name in all lower case letters, and return a
CodecInfo object. In case a search function cannot find
a given encoding, it should return None.

Note

Search function registration is not currently reversible,
which may cause problems in some cases, such as unit testing or
module reloading.

While the builtin open() and the associated io module are the
recommended approach for working with encoded text files, this module
provides additional utility functions and classes that allow the use of a
wider range of codecs when working with binary files:

Open an encoded file using the given mode and return an instance of
StreamReaderWriter, providing transparent encoding/decoding.
The default file mode is 'r', meaning to open the file in read mode.

Note

Underlying encoded files are always opened in binary mode.
No automatic conversion of '\n' is done on reading and writing.
The mode argument may be any binary mode acceptable to the built-in
open() function; the 'b' is automatically added.

encoding specifies the encoding which is to be used for the file.
Any encoding that encodes to and decodes from bytes is allowed, and
the data types supported by the file methods depend on the codec used.

errors may be given to define the error handling. It defaults to 'strict'
which causes a ValueError to be raised in case an encoding error occurs.

buffering has the same meaning as for the built-in open() function.
It defaults to -1 which means that the default buffer size will be used.

Return a StreamRecoder instance, a wrapped version of file
which provides transparent transcoding. The original file is closed
when the wrapped version is closed.

Data written to the wrapped file is decoded according to the given
data_encoding and then written to the original file as bytes using
file_encoding. Bytes read from the original file are decoded
according to file_encoding, and the result is encoded
using data_encoding.

If file_encoding is not given, it defaults to data_encoding.

errors may be given to define the error handling. It defaults to
'strict', which causes ValueError to be raised in case an encoding
error occurs.

Uses an incremental encoder to iteratively encode the input provided by
iterator. This function is a generator.
The errors argument (as well as any
other keyword argument) is passed through to the incremental encoder.

This function requires that the codec accept text str objects
to encode. Therefore it does not support bytes-to-bytes encoders such as
base64_codec.

Uses an incremental decoder to iteratively decode the input provided by
iterator. This function is a generator.
The errors argument (as well as any
other keyword argument) is passed through to the incremental decoder.

This function requires that the codec accept bytes objects
to decode. Therefore it does not support text-to-text encoders such as
rot_13, although rot_13 may be used equivalently with
iterencode().

The module also provides the following constants which are useful for reading
and writing to platform dependent files:

These constants define various byte sequences,
being Unicode byte order marks (BOMs) for several encodings. They are
used in UTF-16 and UTF-32 data streams to indicate the byte order used,
and in UTF-8 as a Unicode signature. BOM_UTF16 is either
BOM_UTF16_BE or BOM_UTF16_LE depending on the platform’s
native byte order, BOM is an alias for BOM_UTF16,
BOM_LE for BOM_UTF16_LE and BOM_BE for
BOM_UTF16_BE. The others represent the BOM in UTF-8 and UTF-32
encodings.

The codecs module defines a set of base classes which define the
interfaces for working with codec objects, and can also be used as the basis
for custom codec implementations.

Each codec has to define four interfaces to make it usable as codec in Python:
stateless encoder, stateless decoder, stream reader and stream writer. The
stream reader and writers typically reuse the stateless encoder/decoder to
implement the file protocols. Codec authors also need to define how the
codec will handle encoding and decoding errors.

To simplify and standardize error handling,
codecs may implement different error handling schemes by
accepting the errors string argument. The following string values are
defined and implemented by all standard Python codecs:

On decoding, replace byte with individual
surrogate code ranging from U+DC80 to
U+DCFF. This code will then be turned
back into the same byte when the
'surrogateescape' error handler is used
when encoding the data. (See PEP 383 for
more.)

In addition, the following error handler is specific to the given codecs:

Value

Codecs

Meaning

'surrogatepass'

utf-8, utf-16, utf-32,
utf-16-be, utf-16-le,
utf-32-be, utf-32-le

Allow encoding and decoding of surrogate
codes. These codecs normally treat the
presence of surrogates as an error.

New in version 3.1: The 'surrogateescape' and 'surrogatepass' error handlers.

Changed in version 3.4: The 'surrogatepass' error handlers now works with utf-16* and utf-32* codecs.

New in version 3.5: The 'namereplace' error handler.

Changed in version 3.5: The 'backslashreplace' error handlers now works with decoding and
translating.

The set of allowed values can be extended by registering a new named error
handler:

Register the error handling function error_handler under the name name.
The error_handler argument will be called during encoding and decoding
in case of an error, when name is specified as the errors parameter.

For encoding, error_handler will be called with a UnicodeEncodeError
instance, which contains information about the location of the error. The
error handler must either raise this or a different exception, or return a
tuple with a replacement for the unencodable part of the input and a position
where encoding should continue. The replacement may be either str or
bytes. If the replacement is bytes, the encoder will simply copy
them into the output buffer. If the replacement is a string, the encoder will
encode the replacement. Encoding continues on original input at the
specified position. Negative position values will be treated as being
relative to the end of the input string. If the resulting position is out of
bound an IndexError will be raised.

Decoding and translating works similarly, except UnicodeDecodeError or
UnicodeTranslateError will be passed to the handler and that the
replacement from the error handler will be put into the output directly.

Previously registered error handlers (including the standard error handlers)
can be looked up by name:

Encodes the object input and returns a tuple (output object, length consumed).
For instance, text encoding converts
a string object to a bytes object using a particular
character set encoding (e.g., cp1252 or iso-8859-1).

The errors argument defines the error handling to apply.
It defaults to 'strict' handling.

The method may not store state in the Codec instance. Use
StreamWriter for codecs which have to keep state in order to make
encoding efficient.

The encoder must be able to handle zero length input and return an empty object
of the output object type in this situation.

Decodes the object input and returns a tuple (output object, length
consumed). For instance, for a text encoding, decoding converts
a bytes object encoded using a particular
character set encoding to a string object.

For text encodings and bytes-to-bytes codecs,
input must be a bytes object or one which provides the read-only
buffer interface – for example, buffer objects and memory mapped files.

The errors argument defines the error handling to apply.
It defaults to 'strict' handling.

The method may not store state in the Codec instance. Use
StreamReader for codecs which have to keep state in order to make
decoding efficient.

The decoder must be able to handle zero length input and return an empty object
of the output object type in this situation.

The IncrementalEncoder and IncrementalDecoder classes provide
the basic interface for incremental encoding and decoding. Encoding/decoding the
input isn’t done with one call to the stateless encoder/decoder function, but
with multiple calls to the
encode()/decode() method of
the incremental encoder/decoder. The incremental encoder/decoder keeps track of
the encoding/decoding process during method calls.

The joined output of calls to the
encode()/decode() method is
the same as if all the single inputs were joined into one, and this input was
encoded/decoded with the stateless encoder/decoder.

The IncrementalEncoder class is used for encoding an input in multiple
steps. It defines the following methods which every incremental encoder must
define in order to be compatible with the Python codec registry.

The errors argument will be assigned to an attribute of the same name.
Assigning to this attribute makes it possible to switch between different error
handling strategies during the lifetime of the IncrementalEncoder
object.

Return the current state of the encoder which must be an integer. The
implementation should make sure that 0 is the most common
state. (States that are more complicated than integers can be converted
into an integer by marshaling/pickling the state and encoding the bytes
of the resulting string into an integer).

The IncrementalDecoder class is used for decoding an input in multiple
steps. It defines the following methods which every incremental decoder must
define in order to be compatible with the Python codec registry.

The errors argument will be assigned to an attribute of the same name.
Assigning to this attribute makes it possible to switch between different error
handling strategies during the lifetime of the IncrementalDecoder
object.

Decodes object (taking the current state of the decoder into account)
and returns the resulting decoded object. If this is the last call to
decode()final must be true (the default is false). If final is
true the decoder must decode the input completely and must flush all
buffers. If this isn’t possible (e.g. because of incomplete byte sequences
at the end of the input) it must initiate error handling just like in the
stateless case (which might raise an exception).

Return the current state of the decoder. This must be a tuple with two
items, the first must be the buffer containing the still undecoded
input. The second must be an integer and can be additional state
info. (The implementation should make sure that 0 is the most common
additional state info.) If this additional state info is 0 it must be
possible to set the decoder to the state which has no input buffered and
0 as the additional state info, so that feeding the previously
buffered input to the decoder returns it to the previous state without
producing any output. (Additional state info that is more complicated than
integers can be converted into an integer by marshaling/pickling the info
and encoding the bytes of the resulting string into an integer.)

All stream writers must provide this constructor interface. They are free to add
additional keyword arguments, but only the ones defined here are used by the
Python codec registry.

The stream argument must be a file-like object open for writing
text or binary data, as appropriate for the specific codec.

The StreamWriter may implement different error handling schemes by
providing the errors keyword argument. See Error Handlers for
the standard error handlers the underlying stream codec may support.

The errors argument will be assigned to an attribute of the same name.
Assigning to this attribute makes it possible to switch between different error
handling strategies during the lifetime of the StreamWriter object.

All stream readers must provide this constructor interface. They are free to add
additional keyword arguments, but only the ones defined here are used by the
Python codec registry.

The stream argument must be a file-like object open for reading
text or binary data, as appropriate for the specific codec.

The StreamReader may implement different error handling schemes by
providing the errors keyword argument. See Error Handlers for
the standard error handlers the underlying stream codec may support.

The errors argument will be assigned to an attribute of the same name.
Assigning to this attribute makes it possible to switch between different error
handling strategies during the lifetime of the StreamReader object.

The set of allowed values for the errors argument can be extended with
register_error().

The chars argument indicates the number of decoded
code points or bytes to return. The read() method will
never return more data than requested, but it might return less,
if there is not enough available.

The size argument indicates the approximate maximum
number of encoded bytes or code points to read
for decoding. The decoder can modify this setting as
appropriate. The default value -1 indicates to read and decode as much as
possible. This parameter is intended to
prevent having to decode huge files in one step.

The firstline flag indicates that
it would be sufficient to only return the first
line, if there are decoding errors on later lines.

The method should use a greedy read strategy meaning that it should read
as much data as is allowed within the definition of the encoding and the
given size, e.g. if optional encoding endings or state markers are
available on the stream, these should be read too.

Creates a StreamReaderWriter instance. stream must be a file-like
object. Reader and Writer must be factory functions or classes providing the
StreamReader and StreamWriter interface resp. Error handling
is done in the same way as defined for the stream readers and writers.

Creates a StreamRecoder instance which implements a two-way conversion:
encode and decode work on the frontend — the data visible to
code calling read() and write(), while Reader and Writer
work on the backend — the data in stream.

You can use these objects to do transparent transcodings from e.g. Latin-1
to UTF-8 and back.

The stream argument must be a file-like object.

The encode and decode arguments must
adhere to the Codec interface. Reader and
Writer must be factory functions or classes providing objects of the
StreamReader and StreamWriter interface respectively.

Error handling is done in the same way as defined for the stream readers and
writers.

Strings are stored internally as sequences of code points in
range 0x0–0x10FFFF. (See PEP 393 for
more details about the implementation.)
Once a string object is used outside of CPU and memory, endianness
and how these arrays are stored as bytes become an issue. As with other
codecs, serialising a string into a sequence of bytes is known as encoding,
and recreating the string from the sequence of bytes is known as decoding.
There are a variety of different text serialisation codecs, which are
collectivity referred to as text encodings.

The simplest text encoding (called 'latin-1' or 'iso-8859-1') maps
the code points 0–255 to the bytes 0x0–0xff, which means that a string
object that contains code points above U+00FF can’t be encoded with this
codec. Doing so will raise a UnicodeEncodeError that looks
like the following (although the details of the error message may differ):
UnicodeEncodeError:'latin-1'codeccan'tencodecharacter'\u1234'inposition3:ordinalnotinrange(256).

There’s another group of encodings (the so called charmap encodings) that choose
a different subset of all Unicode code points and how these code points are
mapped to the bytes 0x0–0xff. To see how this is done simply open
e.g. encodings/cp1252.py (which is an encoding that is used primarily on
Windows). There’s a string constant with 256 characters that shows you which
character is mapped to which byte value.

All of these encodings can only encode 256 of the 1114112 code points
defined in Unicode. A simple and straightforward way that can store each Unicode
code point, is to store each code point as four consecutive bytes. There are two
possibilities: store the bytes in big endian or in little endian order. These
two encodings are called UTF-32-BE and UTF-32-LE respectively. Their
disadvantage is that if e.g. you use UTF-32-BE on a little endian machine you
will always have to swap bytes on encoding and decoding. UTF-32 avoids this
problem: bytes will always be in natural endianness. When these bytes are read
by a CPU with a different endianness, then bytes have to be swapped though. To
be able to detect the endianness of a UTF-16 or UTF-32 byte sequence,
there’s the so called BOM (“Byte Order Mark”). This is the Unicode character
U+FEFF. This character can be prepended to every UTF-16 or UTF-32
byte sequence. The byte swapped version of this character (0xFFFE) is an
illegal character that may not appear in a Unicode text. So when the
first character in an UTF-16 or UTF-32 byte sequence
appears to be a U+FFFE the bytes have to be swapped on decoding.
Unfortunately the character U+FEFF had a second purpose as
a ZEROWIDTHNO-BREAKSPACE: a character that has no width and doesn’t allow
a word to be split. It can e.g. be used to give hints to a ligature algorithm.
With Unicode 4.0 using U+FEFF as a ZEROWIDTHNO-BREAKSPACE has been
deprecated (with U+2060 (WORDJOINER) assuming this role). Nevertheless
Unicode software still must be able to handle U+FEFF in both roles: as a BOM
it’s a device to determine the storage layout of the encoded bytes, and vanishes
once the byte sequence has been decoded into a string; as a ZEROWIDTHNO-BREAKSPACE it’s a normal character that will be decoded like any other.

There’s another encoding that is able to encoding the full range of Unicode
characters: UTF-8. UTF-8 is an 8-bit encoding, which means there are no issues
with byte order in UTF-8. Each byte in a UTF-8 byte sequence consists of two
parts: marker bits (the most significant bits) and payload bits. The marker bits
are a sequence of zero to four 1 bits followed by a 0 bit. Unicode characters are
encoded like this (with x being payload bits, which when concatenated give the
Unicode character):

Range

Encoding

U-00000000 … U-0000007F

0xxxxxxx

U-00000080 … U-000007FF

110xxxxx 10xxxxxx

U-00000800 … U-0000FFFF

1110xxxx 10xxxxxx 10xxxxxx

U-00010000 … U-0010FFFF

11110xxx 10xxxxxx 10xxxxxx 10xxxxxx

The least significant bit of the Unicode character is the rightmost x bit.

As UTF-8 is an 8-bit encoding no BOM is required and any U+FEFF character in
the decoded string (even if it’s the first character) is treated as a ZEROWIDTHNO-BREAKSPACE.

Without external information it’s impossible to reliably determine which
encoding was used for encoding a string. Each charmap encoding can
decode any random byte sequence. However that’s not possible with UTF-8, as
UTF-8 byte sequences have a structure that doesn’t allow arbitrary byte
sequences. To increase the reliability with which a UTF-8 encoding can be
detected, Microsoft invented a variant of UTF-8 (that Python 2.5 calls
"utf-8-sig") for its Notepad program: Before any of the Unicode characters
is written to the file, a UTF-8 encoded BOM (which looks like this as a byte
sequence: 0xef, 0xbb, 0xbf) is written. As it’s rather improbable
that any charmap encoded file starts with these byte values (which would e.g.
map to

LATIN SMALL LETTER I WITH DIAERESIS

RIGHT-POINTING DOUBLE ANGLE QUOTATION MARK

INVERTED QUESTION MARK

in iso-8859-1), this increases the probability that a utf-8-sig encoding can be
correctly guessed from the byte sequence. So here the BOM is not used to be able
to determine the byte order used for generating the byte sequence, but as a
signature that helps in guessing the encoding. On encoding the utf-8-sig codec
will write 0xef, 0xbb, 0xbf as the first three bytes to the file. On
decoding utf-8-sig will skip those three bytes if they appear as the first
three bytes in the file. In UTF-8, the use of the BOM is discouraged and
should generally be avoided.

Python comes with a number of codecs built-in, either implemented as C functions
or with dictionaries as mapping tables. The following table lists the codecs by
name, together with a few common aliases, and the languages for which the
encoding is likely used. Neither the list of aliases nor the list of languages
is meant to be exhaustive. Notice that spelling alternatives that only differ in
case or use a hyphen instead of an underscore are also valid aliases; therefore,
e.g. 'utf-8' is a valid alias for the 'utf_8' codec.

CPython implementation detail: Some common encodings can bypass the codecs lookup machinery to
improve performance. These optimization opportunities are only
recognized by CPython for a limited set of (case insensitive)
aliases: utf-8, utf8, latin-1, latin1, iso-8859-1, iso8859-1, mbcs
(Windows only), ascii, us-ascii, utf-16, utf16, utf-32, utf32, and
the same using underscores instead of dashes. Using alternative
aliases for these encodings may result in slower execution.

Changed in version 3.6: Optimization opportunity recognized for us-ascii.

Many of the character sets support the same languages. They vary in individual
characters (e.g. whether the EURO SIGN is supported or not), and in the
assignment of characters to code positions. For the European languages in
particular, the following variants typically exist:

an ISO 8859 codeset

a Microsoft Windows code page, which is typically derived from an 8859 codeset,
but replaces control characters with additional graphic characters

Changed in version 3.4: The utf-16* and utf-32* encoders no longer allow surrogate code points
(U+D800–U+DFFF) to be encoded.
The utf-32* decoders no longer decode
byte sequences that correspond to surrogate code points.

A number of predefined codecs are specific to Python, so their codec names have
no meaning outside Python. These are listed in the tables below based on the
expected input and output types (note that while text encodings are the most
common use case for codecs, the underlying codec infrastructure supports
arbitrary data transforms rather than just text encodings). For asymmetric
codecs, the stated purpose describes the encoding direction.

This module implements RFC 3490 (Internationalized Domain Names in
Applications) and RFC 3492 (Nameprep: A Stringprep Profile for
Internationalized Domain Names (IDN)). It builds upon the punycode encoding
and stringprep.

These RFCs together define a protocol to support non-ASCII characters in domain
names. A domain name containing non-ASCII characters (such as
www.Alliancefrançaise.nu) is converted into an ASCII-compatible encoding
(ACE, such as www.xn--alliancefranaise-npb.nu). The ACE form of the domain
name is then used in all places where arbitrary characters are not allowed by
the protocol, such as DNS queries, HTTP Host fields, and so
on. This conversion is carried out in the application; if possible invisible to
the user: The application should transparently convert Unicode domain labels to
IDNA on the wire, and convert back ACE labels to Unicode before presenting them
to the user.

Python supports this conversion in several ways: the idna codec performs
conversion between Unicode and ACE, separating an input string into labels
based on the separator characters defined in section 3.1 of RFC 3490
and converting each label to ACE as required, and conversely separating an input
byte string into labels based on the . separator and converting any ACE
labels found into unicode. Furthermore, the socket module
transparently converts Unicode host names to ACE, so that applications need not
be concerned about converting host names themselves when they pass them to the
socket module. On top of that, modules that have host names as function
parameters, such as http.client and ftplib, accept Unicode host
names (http.client then also transparently sends an IDNA hostname in the
Host field if it sends that field at all).

When receiving host names from the wire (such as in reverse name lookup), no
automatic conversion to Unicode is performed: Applications wishing to present
such host names to the user should decode them to Unicode.

The module encodings.idna also implements the nameprep procedure, which
performs certain normalizations on host names, to achieve case-insensitivity of
international domain names, and to unify similar characters. The nameprep
functions can be used directly if desired.

This module implements a variant of the UTF-8 codec: On encoding a UTF-8 encoded
BOM will be prepended to the UTF-8 encoded bytes. For the stateful encoder this
is only done once (on the first write to the byte stream). For decoding an
optional UTF-8 encoded BOM at the start of the data will be skipped.